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I can't get my head around it completely. I am standing on an Earth-sized planet circling a black hole. Let's assume the planet to be an incompressible one.

It is observable that matter is sucked in black holes from stars orbiting around them. At a certain distance though this will not happen anymore. The stuff of the star will stay within the star.

What will I feel on distances for which I will not be sucked in the hole? Will I just feel stretched out? Is this stretching gettting stronger up to the point I get sucked in (when the distance grows smaller)?

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    $\begingroup$ "sucked in" is not how black holes work. They just have gravity. Ordinary common-or-garden general relativity gravity. It doesn't suck any more than the Earth is sucking you to the floor. $\endgroup$
    – James K
    Jun 29, 2021 at 20:14
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    $\begingroup$ Tidal forces happen whatever the object a planet is orbiting. $\endgroup$
    – ProfRob
    Jun 30, 2021 at 7:07
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    $\begingroup$ If you can "feel" the tidal forces then you won't be standing on an intact planet. $\endgroup$
    – ProfRob
    Jun 30, 2021 at 8:05
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    $\begingroup$ Stop adding further questions to your original question. $\endgroup$
    – ProfRob
    Jun 30, 2021 at 10:45
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    $\begingroup$ Deschele, I'm not sure this question can be rescued from closure, and you've already had another similar question closed. Is your main interest whether a planet can orbit a BH? See this question. If you're more interested in whether such a planet could be habitable, see this question. If neither of these address your concern, what are you looking to find out? :-) $\endgroup$
    – Chappo Hasn't Forgotten Monica
    Jul 1, 2021 at 0:31

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For a planet like the Earth, it is reasonably straightforward to show that the tidal acceleration across the planet is around $1g$ at the innermost stable circular orbit (ISCO) of a $\sim 10^8 M_\odot$ black hole (i.e. an orbit of 3 Schwarzschild radii in size).

Since a person is about 1/6400000 of the extent of the planet, then stretching acceleration they experience, which is proportional to the extent of an object, will be just $1/6400000$ of $1g$. This would not be felt.

If you orbit a less massive black hole at its ISCO, then the tidal forces get bigger. But the planet would then be ripped apart, because that's what happens if the tidal acceleration exceeds the surface gravity. There is no configuration where you can stand on an intact planet (of a sensible size) and experience a direct tidal force.

If you are standing on a planet that falls toward a black hole, the planet will disintegrate before you begin to feel any tidal stretching.

In fact it is also a straightforward calculation to show that, for a free falling person, the time from when the stretching becomes noticeable to reaching the singularity is of order a few tenths of a second; about the time it takes sensations to reach and be processed by the brain, irrespective of the black hole mass.

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  • $\begingroup$ What if the planet were incompressable? Almost like a neutron star. If it has a very small radius ( but the same mass as the Earth) will I stay or will I go? Will I find myself on a Lagragian point at a certain distane so I dont feel gravitational effects (except tidal forces)? $\endgroup$
    – Deschele Schilder
    Jun 30, 2021 at 9:42
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    $\begingroup$ What if the planet was "like a neutron star"? Then you wouldn't be standing on it you would be smeared over the surface. The compressibility of the planet has nothing to do with it. BTW Neutron stars are quite compressible and have an average equation of state like $P \propto \rho^2$ which is much more compressible than say the Earth. $\endgroup$
    – ProfRob
    Jun 30, 2021 at 9:53
  • $\begingroup$ Will there not be a distance where I dont feel the gravity of the neutron star? $\endgroup$
    – Deschele Schilder
    Jun 30, 2021 at 10:00
  • $\begingroup$ @DescheleSchilder If you feel the tides (which is not gravity, but gravity gradient!), then you are not on a planet anymore, as it means the planet cannot exist there. There are no incompressible planets. $\endgroup$
    – planetmaker
    Jun 30, 2021 at 10:05
  • $\begingroup$ Event horizons are not material. You may pass them without noticing or any adverse effects. It's just the point of no return. Whether tides are dangerous there depends on the mass of the black hole it belongs to. $\endgroup$
    – planetmaker
    Jun 30, 2021 at 11:38
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If you were to swap the sun with a black hole of exactly the same mass, the only thing you’d feel different is cold. The Earth would go along its slightly elliptical orbits as if nothing happened, as the new black hole and the old sun would have the same gravitational effects.

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Black holes do not "suck" a planet in, rather, think of them as the sun with an insanely multiplied gravitational force, constantly changing your orbit until it isn't even an orbit, rather a suborbit. Once the planet is within extremely close proximity (like a spacecraft reentering when coming back to Earth), the gravity alters the structural stability of a planet (Sorta like the Kraken if you play KSP) and then rips it apart. While orbiting the tidal forces would only be an extremely small fraction of a g, and you would be disintegrated before your body would be able to perceive the tidal forces. So yeah, in other words you woudn't really experience anything because you would die first...

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    $\begingroup$ Hi Linus, welcome to Stack Exchange. Please note that we're not like a discussion forum where users post a range of personal opinions; on SE we're looking for detailed and authoritative answers. We assess this by voting; the Help Centre says "Voting up a question or answer signals to the rest of the community that a post is interesting, well-researched, and useful, while voting down a post signals the opposite." See the earlier, more detailed answer as an example of what gets upvoted. For further guidance, see How to Answer and take our Tour. :-) $\endgroup$
    – Chappo Hasn't Forgotten Monica
    Jun 30, 2021 at 23:40

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